A Detailed Rainfall Analysis and Probabilistic Perspective of Hurricane Harvey’s Extraordinary Rainfall

The large areal extent and high storm efficiency across a range of durations made the rainfall from hurricane Harvey in the Houston, Texas area extraordinary. In order to objectively convey the rareness of the rainfall from Harvey, the rainfall depths for several durations (6-, 24-, 72-, and 120-hours) were translated into an equivalent annual exceedance probabilities (AEP) or “return period.” Current precipitation frequency documents from NOAA’s National Weather Service only provide frequencies out to 100-years. Harvey’s rainfall far exceeded a 100-year event, so to attain more reliable AEPs beyond 100-years, a new (2016) Texas precipitation analysis created by MetStat was utilized. Additionally, a 1998 United State Geological Survey report was used to compute long duration (>24 hour) AEPs well past 1,000-years. Coupling these precipitation frequency resources with a detailed MetStorm precipitation analysis, based on radar data and over 2,000 rain gauge reports, resulted in some shocking statistics and maps. For a 120-hour period, our analysis suggests large areas experienced rainfalls well over a 1,000-year event; so rare, that the storms’ efficiency reached theoretical limits of precipitation production. In fact, for long durations-large area combinations, the rainfall depths exceeded Probable Maximum Precipitation (PMP), which is the theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year. PMP is used for the safe design and operation of major infrastructure, such as dams and nuclear power plants, which for the most part weathered the storm in Houston. Harvey represents the only known storm to clearly exceed PMP in the United States. This presentation will provide a detailed rainfall analysis, a probabilistic perspective of the rainfall associated with Harvey and innovative ways to effectively communicate this and future extreme precipitation events.